1. Field of the Invention
The present invention relates to a foldable optical apparatus, for example an overhead projector, and more particularly relates to an optical apparatus of improved portability.
2. Description Related to the Prior Art
There is a known optical apparatus such as an overhead projector, which is either of a reflection type or a transmission type. Such an overhead projector has a stage. A projecting head is held above the stage and substantially parallel to it, and includes an optical system. On the stage, there is placed an original material, namely a transparency on which an opaque image is written or drawn to be projected.
In a reflection type, the stage is constituted by a Fresnel mirror consisting of a Fresnel plate and a reflective layer formed thereon. A light source is included in the projecting head. To project the image on a screen remote from the apparatus, light emitted by the light source is directed to the Fresnel mirror. Transmitted through the original material, the light is reflected upward by the Fresnel mirror to illuminate the original material from below. The light of the image of the original material passes through the optical system inclusive of a projector lens and a projector mirror, and proceeds to the screen.
In a transmission type of overhead projector, a stage is constituted by a Fresnel lens. A light source is contained under the stage. To project the image, light emitted by the light source is passed through the Fresnel lens, and through the original material. The light of the image of the original material passes through an optical system in the projecting head, and proceeds to the screen.
As is suggested in U.S. Pat. No. 5,317,350, the overhead projector, in general, has a base table having the stage, on which a support or a stand is mounted in swingable fashion for the purpose of portability. On a distal end of the stand, a projecting head is mounted. To collapse the overhead projector, the stand and the projecting head are folded to lie on the base table. It would be conceivable to construct an overhead projector in which a projecting head would be supported swingably on a stand, and in which the projecting head could be set up over a stage in response to erection of the stand.
The projecting head on the top of the stand has somewhat great weight. To swing the projecting head in association with the erection of the stand, sufficiently great force must be applied to the stand in the erecting direction. In view of reducing such an erecting force, it is known to associate a spring or other bias devices with the stand, for biasing the stand in the erecting direction.
Only a single spring is used for biasing the stand in an erecting direction. It follows that the biasing force received by stand from the spring is changed remarkably within the range of swing of the stand. The biasing force could be set to increase in the erecting direction, for facility in positioning the stand in the erected position. However, the stand to be erected would require manual application of excessively great force in an initial step of the erection. The upwardly increasing biasing force would cause a difficulty in initially unfolding the stand. Otherwise biasing force of a different spring could be set to decrease in the erecting direction, for retaining the stand folded with ease. However, the stand during erection would require excessively great force in a final step of the erection.